Build rules exercise

In your Chromium build, add a new Rust target to //ui/base/BUILD.gn containing:

#![allow(unused)]
fn main() {
// SAFETY: There is no other global function of this name.
#[unsafe(no_mangle)]
pub extern "C" fn hello_from_rust() {
    println!("Hello from Rust!")
}
}

Important: note that no_mangle here is considered a type of unsafety by the Rust compiler, so you’ll need to allow unsafe code in your gn target.

Add this new Rust target as a dependency of //ui/base:base. Declare this function at the top of ui/base/resource/resource_bundle.cc (later, we’ll see how this can be automated by bindings generation tools):

extern "C" void hello_from_rust();

Call this function from somewhere in ui/base/resource/resource_bundle.cc - we suggest the top of ResourceBundle::MaybeMangleLocalizedString. Build and run Chromium, and ensure that “Hello from Rust!” is printed lots of times.

If you use VSCode, now set up Rust to work well in VSCode. It will be useful in subsequent exercises. If you’ve succeeded, you will be able to use right-click “Go to definition” on println!.

Where to find help

• The options available to the rust_static_library gn template
• Information about #[unsafe(no_mangle)]
• Information about extern “C”
• Information about gn’s --export-rust-project switch
• How to install rust-analyzer in VSCode

It’s really important that students get this running, because future exercises will build on it.

This example is unusual because it boils down to the lowest-common-denominator interop language, C. Both C++ and Rust can natively declare and call C ABI functions. Later in the course, we’ll connect C++ directly to Rust.

allow_unsafe = true is required here because #[unsafe(no_mangle)] might allow Rust to generate two functions with the same name, and Rust can no longer guarantee that the right one is called.

If you need a pure Rust executable, you can also do that using the rust_executable gn template.